This invention relates to the production of tantalum, niobium, and other metal powders by the reduction of the corresponding metal oxide with gaseous active metals such as Mg, Ca, and other elemental and compound reducing materials, in gaseous form.
Tantalum and niobium are members of a group of metals that are difficult to isolate in the free state because of the stability of their compounds, especially the oxide. A review of the methods developed to produce tantalum will serve to illustrate the history of a typical manufacturing process for these metals. Tantalum metal powder was first produced on a commercial scale in Germany at the beginning of the 20th Century by the reduction of the double salt, potassium heptafluorotantalate (K.sub.2 TaF.sub.7) with sodium. Small pieces of sodium were mixed with the tantalum containing salt and sealed into a steel tube. The tube was heated at the top with a ring burner and, after ignition, the reduction reaction proceeded quickly down the tube. The reaction mixture was allowed to cool and the solid mass, consisting of tantalum metal powder, unreacted K.sub.2 TaF.sub.7 and sodium, and other products of the reduction was removed by hand using a chisel. The mixture was crushed and then leached with dilute acid to separate the tantalum from the other components. The process was difficult to control, dangerous, and produced a coarse, contaminated powder, but nevertheless pointed the way to what became the principal means of production of high purity tantalum in later years.
Commercial production of tantalum metal in the United States began in the 1930's. A molten mixture of K.sub.2 TaF.sub.7 containing tantalum oxide (Ta.sub.2 O.sub.5) was electrolyzed at 700.degree. C. in a steel retort. When the reduction reaction was completed, the system was cooled and the solid mass removed from the electrolysis cell, and then crushed and leached to separate the coarse tantalum powder from the other reaction products. The dendritic powder was not suitable for use directly in capacitor applications.
The modern method for manufacturing tantalum was developed in the late 1950's by Hellier and Martin..sup.1 Following Hellier and Martin, and hundreds of subsequently described implementations or variations, a molten mixture of K.sub.2 TaF.sub.7 and a diluent salt, typically NaCl, is reduced with molten sodium in a stirred reactor. Using this system, control of the important reaction variables, such as reduction temperature, reaction rate, and reaction composition, was feasible. Over the years, the process was refined and perfected to the point where high quality powders with surface area exceeding 20,000 cm.sup.2 /gm are produced and materials with surface area in the 5000-8000 cm.sup.2 /gm range being typical. The manufacturing process still requires the removal of the solid reaction products from the retort, separation of the tantalum powder from the salts by leaching, and treatments like agglomeration to improve the physical properties. Most capacitor grade tantalum powders are also deoxidized with magnesium to minimize the oxygen content..sup.2 Artifacts of preagglomeration of primary particles to secondary particle form and doping with materials to enhance capacitance (e.g. P, N, Si, C) are also known today.
 FNT .sup.1 Hellier, E. G. and Martin, G. L., U.S. Pat. No. 2,950,185, 1960.
 FNT .sup.2 Albrecht, W. W., Hoppe, H., Papp, V. and Wolf, R., U.S. Pat. No. 4,537,641, 1985.
While the reduction of K.sub.2 TaF.sub.7 with sodium has allowed the industry to make high performance, high quality tantalum powders, there are several drawbacks to this method. It is a batch process prone to the inherent variability in the system; as a result, batch to batch consistency is difficult. Post reduction processing (mechanical and hydrometallurgical separations, filtering) is complex, requiring considerable human and capital resources and it is time consuming. The disposal of large quantities of reaction products containing fluorides and chlorides can be a problem. Of fundamental significance, the process has evolved to a state of maturity such that significant advances in the performance of the tantalum powder produced is unlikely.
Over the years, numerous attempts were made to develop alternate ways for reducing tantalum and similar metal compounds to the metallic state..sup.3 Among these were the use of active metals other than sodium, like calcium, magnesium, and aluminum and raw materials such as tantalum pentoxide and tantalum chloride. As seen in Table I, the negative Gibbs free energy changes mean that the reduction of the oxides of Ta, Nb, and other metals with magnesium to the metallic state is favorable; reaction rate and method determine the feasibility of using this approach to produce high quality powders on a commercial scale. To date, none of these approaches were commercialized significantly because they did not produce high quality powders. Apparently, the reason this approach failed in the past was because the reductions were carried out by blending the reducing agent with the metal oxide. The reaction took place in contact with the molten reducing agent and this condition adversely affected the morphology and even the chemistry of the products.
 FNT .sup.3 Miller, G. L.,. "Tantalum and Niobium", London, 1959, pp. 189-94; Marden, J. W. and Rich, M. H., U.S. Pat. No. 1,728,941, 1927; and Gardner, D., U.S. Pat. No. 2,516,863, 1946.
TABLE I Gibbs Free Energy Change for Reduction of Metal Oxides with Magnesium M.sub.x O.sub.y (s) + yMg(g) .fwdarw. yMgO(s) + xM(s) Temperature Gibbs Free Energy Change (Kcal/mole oxide) .degree. C. Ta.sub.2 O.sub.5 Nb.sub.2 O.sub.5 TiO.sub.2 V.sub.2 O.sub.3 ZrO.sub.2 WO.sub.2 200 -219 -254 -58 -133 -22 -143 400 -215 -249 -56 -130 -21 -141 600 -210 -244 -55 -126 -20 -139 800 -202 -237 -52 -122 -18 -137 1000 -195 -229 -50 -116 -15 -134 1200 -186 -221 -47 -111 -13 -131 1400 -178 -212 -45 -106 -11 -128
The use of magnesium to deoxidize or reduce the oxygen content of tantalum metal is well known..sup.4 The process involves blending the metal powder with 1-3 percent magnesium and heating to achieve the reduction process. The magnesium is in the molten state during a portion of the heating time. In this case, the objective is to remove 1000-3000 ppm oxygen and only a low concentration of MgO is produced. However, when a much greater quantity of tantalum oxide is reduced a large quantity of magnesium oxide is generated. The resulting mixture of magnesium, tantalum oxide and magnesium oxide can form tantalum-magnesium-oxygen complexes that are difficult to separate from the tantalum metal.
 FNT .sup.4 See n. 2, above.
It is a principal object of the invention to provide a new approach to production of tantalum and similar metals that provides a means of eliminating one or more, preferrably all, the problems of traditional double salt reduction and follow on processing.
It is a further object of the invention to enable a continuous production process.
It is a further object of the invention to provide improved metal forms.